Presently the state of art with regard to practical devices for measuring and displaying the level of a fluid contained within a tank have taken the form of mechanical devices, i.e., based on the displacement of a float or other analysis movable member, and electrically capacitive devices, i.e., based upon the change in capacitance of an electrical sensor as a result of a liquid or air capacitance link in an electrical circuit.
Attempts have been made to provide a passive system for the detection of a liquid within a tank utilizing elements to receive and redirect light by reflection when surrounded by a medium of different index of refraction than that of a liquid whose level is to be detected. An illustration of such attempts is provided by U.S. Pat. No. 3,120,125.
However, it has been the desire of these prior art attempts to look at the surface of the fluid within the tank along a horizontal plane or several horizontal planes through the tank.
With such designs it has been found that it is extremely difficult to mount the detecting apparatus in close proximity one to the other so as to monitor the small changes in fluid level within the tank and reflections from the top of a fluid can not accurately determine the density of the fluid when there is any disturbances in the fluid.